Hi Paul,
I am a South London (Croydon) cyclist and in July I am cycling Santander to Santiago by the Camino del Norte and returning to Santander via Lugo and the Camino Primitivo. I cannot answer your question about where to collect your second bike in Spain, other than to suggest you send it by rail to a convenient place on your route.
I have cycled Santander/Santiago before, in 2005, so do have some experience. We stayed on the roads which followed nearest to the off road camino tracks.
We are planning on 40 to 50 miles a day, carrying our luggage. Spain is very hilly, and not just in the mountains! I would suggest that when you are able to, you start training with longer rides, and build up by getting used to cycling hills. Croydon is on the edge of the North Downs and with my companions who are cycling with me to Santiago, we are doing a lot of mileage and lots of hills. Since the beginning of the year we have been going out at least twice a week and recently have been trying to get in 3 days. We are now typically doing at least 100 miles a week, 2 weeks ago we actually manage just over 250 miles in 4 days! With this higher mileage I am beginning to lose weight, very useful as you have to carry every lb up every hill!
My bike is a rigid framed mountain bike,at least 10 years old with no front or rear suspension. It does however have 30 gears (triple chain rings and 9 speed cassette), and the very low gears that mountain bikes provide are invaluable for the Spanish terrain.
I might add that I am 72 tears of age, so not exactly a lycra lout! I am however a long time cyclist and member of the Anerley Bicycle Club so have no problem getting training miles in. Being a Londoner, you might like to join us for some enjoyable cycling in preparation for your ride.
To find out a bit about the Anerley Bicycle Club and whether or not it might be to your liking, see
http://anerleybc.org/ Or for an item which I just posted introducing our planning for the ride to santiago, see
http://anerleybc.org/planning-a-cycling-adventure/
Picos Rider.